Heating and ventilating, or air conditioning, installations for the cabins of motor vehicles commonly include air flow control devices comprising a masking member in the form of a pivoting or sliding valve member disposed in a corresponding fluid flow duct. The pivoting or sliding movement of these valve members, and the presence of actuating means for the latter, necessitates quite a large component size in the direction of the fluid flow and/or transversely to that direction, which makes such devices difficult to fit into the restricted space that is available below the fascia of the vehicle.